How is Merkel cell carcinoma detected in the lymph nodes?

• · What is a lymph node?
• · What is a sentinel lymph node biopsy?

MCC can spread through the body (metastasize) via the lymphatic system. It is a system of vessels and lymph nodes throughout the body (see diagram below). The lymphatic circulation serves an important function in the immune system. Lymph nodes act as filters to trap cancer cells as they travel through the lymphatic vessels.

Schematic representation of the lymphatic system. MCC cells can travel from the primary site, through the lymph vessels to the sentinel lymph node. Note that MCC on the leg will likely drain to the inguinal lymph nodes on the same side; a primary on the arm will drain to the axilla (armpit); MCC on the trunk can drain to the closest axilla or inguinal bed, or multiple beds unpredictably; a primary on the face will drain under chin (submandibular) or in front of the ear (pre-auricular).

What is a sentinel lymph node biopsy?

MCC can travel from the skin, through the lymphatic vessels, to the sentinel lymph node. The sentinel lymph node is the first lymph node in which MCC can be found. If a lymph node feels enlarged, it may contain MCC (macro metastases). Sometimes, lymph nodes may contain MCC, but not feel enlarged (micro metastases). Lymph nodes should be removed (biopsied) to determine if MCC is present.

There is a technique to identify the sentinel lymph node when it cannot be felt on physical exam. A blue dye and a radioactive tracer are injected at the site of the primary lesion. Within 5 to 10 minutes, the dye and tracer travel along the same path that cancer cells would spread through the lymphatic vessels and collect in the sentinel lymph node. An instrument that detects the tracer is used to map the path from the skin to the sentinel lymph node. The sentinel lymph node is removed and examined for the presence of MCC. If MCC is not found in the sentinel lymph node, then the chance that it has spread beyond the skin is lower than if micro metastases are present.

This technique has a low risk of significant side effects, provides useful information on the chance of spread, and identifies the lymph node region containing the sentinel lymph node (draining lymph node basin), which is sometimes difficult, especially for lesions on the trunk.

Mapping a sentinel lymph node. A radioactive tracer was injected at the site of a skin cancer on the left flank. The tracer traveled along the lymphatic vessels to a lymph node in the left groin and was then photographed using a special x-ray technique. This procedure allows the surgeon to identify the sentinel lymph node and remove it for pathologic analysis. Adapted from Perrott, 2003, with permission.

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